Drexel, Trinity researchers use MXene to enable silicon anodes to avoid large volume changes under cycling
Researchers from Drexel University and Trinity College in Ireland have shown that two-dimensional titanium carbide or carbonitride nanosheets—MXenes—can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives. The nanosheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450 μm).
Consequently, they report in an open-access paper in the Nature Communications, they have demonstrated very high areal capacity anodes (up to 23.3 mAh cm−2).
Traditionally, electrode additives are made of dual components based on a conductive agent (i.e. carbon black, CB) and a polymeric binder. While the former ensures the charge transport throughout the electrode, the latter mechanically holds the active materials and CB together during cycling. Although these traditional electrode additives have been widely applied in Li-ion battery technologies, they fail to perform well in high-capacity electrodes, especially those displaying large volume changes. This is because the polymeric binder is not mechanically robust enough to withstand the stress induced during lithiation/delithiation, leading to severe disruption of the conducting networks. This results in rapid capacity fade and poor lifetime.
This issue can be solved by employing a conductive binder to accommodate the large volume change of the electrodes. … Here we show that the goals outlined above can be simultaneously achieved by using MXene nanosheets as a new class of conductive binder to fabricate high-M/ASi/MXene anodes without any additional polymer or CB.—Zhang et al.
During the slurry-casting process, sheets of MXene material combine with silicon particles to form a network that allows for a more orderly reception of lithium ions, which prevents the silicon anode from expanding and breaking. Source: Drexel University
Fortifying silicon with MXene could extend the life of Li-ion batteries as much as five times; the two-dimensional MXene material prevents the silicon anode from expanding to its breaking point during charging.
Most solutions to the volumetric expansion problem with silicon anodes have involved adding carbon materials and polymer binders to create a framework to contain the silicon. The process for doing it, according to Yury Gogotsi, PhD, Distinguished University and Bach Professor in Drexel’s College of Engineering and director of the A.J. Drexel Nanomaterials Institute in the Department of Materials Science and Engineering, and co-author of the research, is complex and carbon contributes little to charge storage by the battery.
By contrast, the Drexel and Trinity group’s method mixes silicon powder into a MXene solution to create a hybrid silicon-MXene anode. MXene nanosheets distribute randomly and form a continuous network while wrapping around the silicon particles, thus acting as conductive additive and binder at the same time. It’s the MXene framework that also imposes order on ions as they arrive and prevents the anode from expanding.
MXenes are the key to helping silicon reach its potential in batteries. Because MXenes are two-dimensional materials, there is more room for the ions in the anode and they can move more quickly into it — thus improving both capacity and conductivity of the electrode. They also have excellent mechanical strength, so silicon-MXene anodes are also quite durable up to 450 microns thickness.—Yury Gogotsi
MXenes, which were first discovered at Drexel in 2011, are made by chemically etching a layered ceramic material called a MAX phase, to remove a set of chemically-related layers, leaving a stack of two-dimensional flakes. Researchers have produced more than 30 types of MXene to date, each with a slightly different set of properties. The group selected two of them to make the silicon-MXene anodes tested for the paper: titanium carbide and titanium carbonitride. They also tested battery anodes made from graphene-wrapped silicon nanoparticles.
All three anode samples showed higher lithium-ion capacity than current graphite or silicon-carbon anodes used in Li-ion batteries and superior conductivity—on the order of 100 to 1,000 times higher than conventional silicon anodes, when MXene is added.
The continuous network of MXene nanosheets not only provides sufficient electrical conductivity and free space for accommodating the volume change but also well resolves the mechanical instability of Si. Therefore, the combination of viscous MXene ink and high-capacity Si demonstrated here offers a powerful technique to construct advanced nanostructures with exceptional performance.—Zhang et al.
Chuanfang Zhang, PhD, a post-doctoral researcher at Trinity and lead author of the study, also notes that the production of the MXene anodes, by slurry-casting, is easily scalable for mass production of anodes of any size, which means they could make their way into batteries that power just about any of our devices.
The study was led by Zhang, a post-doctoral researcher at Trinity College who was a PhD student in Gogotsi’s lab. It was a collaborative effort between Gogotsi and Trinity professors Jonathan N. Coleman and Valeria Nicolosi, recognized European leaders in the field of 2D materials. Sang-Hoon Park, Andrés Seral-Ascaso, Sebastian Barwich, Niall McEvoy, Conor S. Boland, from Trinity College, also contributed to this research.
Chuanfang (John) Zhang, Sang-Hoon Park, Andrés Seral‐Ascaso, Sebastian Barwich, Niall McEvoy, Conor S. Boland, Jonathan N. Coleman, Yury Gogotsi & Valeria Nicolosi (2019) “High capacity silicon anodes enabled by MXene viscous aqueous ink” Nature Communications doi: 10.1038/s41467-019-08383-y